Silver nanoparticles decorated ZnO–CuO core–shell nanowire arrays with low water adhesion and high antibacterial activity

Nanostructured surfaces based on silver nanoparticles decorated ZnO–CuO core–shell nanowire arrays, which can assure protection against various environmental factors such as water and bacteria were developed by combining dry preparation techniques namely thermal oxidation in air, radio frequency (RF) magnetron sputtering and thermal vacuum evaporation. Thus, high-aspect-ratio ZnO nanowire arrays were grown directly on zinc foils by thermal oxidation in air. Further ZnO nanowires were coated with a CuO layer by RF magnetron sputtering, the obtained ZnO–CuO core–shell nanowires being decorated with Ag nanoparticles by thermal vacuum evaporation. The prepared samples were comprehensively assessed from morphological, compositional, structural, optical, surface chemistry, wetting and antibacterial activity point of view. The wettability studies show that native Zn foil and ZnO nanowire arrays grown on it are featured by a high water droplet adhesion while ZnO–CuO core–shell nanowire arrays (before and after decoration with Ag nanoparticles) reveal a low water droplet adhesion. The antibacterial tests carried on Escherichia coli (a Gram-negative bacterium) and Staphylococcus aureus (a Gram-positive bacterium) emphasize that the nanostructured surfaces based on nanowire arrays present excellent antibacterial activity against both type of bacteria. This study proves that functional surfaces obtained by relatively simple and highly reproducible preparation techniques that can be easily scaled to large area are very attractive in the field of water repellent coatings with enhanced antibacterial function.

Semiconducting nanowires are one of the most outstanding one-dimensional nanomaterials being featured by a high surface to volume ratio and tunable physicochemical characteristics, which are making them very attractive for a wide range of applications including (opto-) electronics, photovoltaic cells, (photo-) catalysis, (bio-) sensing, (bio-) medicine, surfaces with controlled wettability, and so on 1,2 .
Metal oxide nanowire arrays based on ZnO and CuO can be easily grown by thermal oxidation in air directly on zinc foils 9,14,16,17 and copper foils 8,10,13,16 , respectively. Thermal oxidation in air is a straightforward and environmental friendly preparation path, which can be adapted for large-scale production of metal oxide nanowires. This technique can be also regarded as a viable solution to micro/nanostructuring a metallic surface, specific properties and new functionalities, different from those presented by the pristine metal bulk material, being www.nature.com/scientificreports/ the visual/color detail being specific for each preparation step. Thus, the Zn foil lost the metallic luster acquiring the typical white color of ZnO after the thermal oxidation, dark brown color after the coating with CuO layer and light brown color after the decoration with Ag nanoparticles. Also, it can be noticed that the flat aspect of the foil is changed into a wrinkled one after the thermal oxidation process.

Characterization techniques.
The prepared samples were characterized by complementary techniques.
The surface morphology and its elemental composition were studied using a Zeiss Merlin Compact field emission scanning electron microscope (FESEM) and a Zeiss EVO 50XVP scanning electron microscope equipped with an energy dispersive X-ray analysis (EDX) QUANTAX Bruker 200 accessory for composition. Furthermore, the morphology, atomic structure and local chemical composition structure at nanoscale level were analyzed by transmission electron microscopy (TEM), EDX including elemental mapping in scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED) using a Cs probe-corrected JEM ARM 200F analytical electron microscope. The crystalline phase was investigated using a Bruker AXS D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm), the source being operated at 40 kV and 40 mA. The reflectance and photoluminescence (PL) were evaluated using a Perkin-Elmer Lambda 45 UV-VIS spectrophotometer equipped with an integrating sphere for reflectance and a FL 920 Edinburgh Instruments spectrometer with a 450 W Xe lamp excitation and double monochromators on both excitation and emission for photoluminescence (λ exc = 350 nm), respectively. The surface chemistry was analyzed by X-Ray photoelectron spectroscopy (XPS) with an AXIS Ultra DLD (Kratos Surface Analysis) setup using Al K α1 (1486.74 eV) radiation produced by a monochromatized X-Ray source at operating power of 225 W (15 kV × 15 mA). In the analysis chamber, the base pressure was at least 1.0 × 10 -8 mbar. A flood gun operating at 1.5 A filament current, 2.7 V charge balance and 1.0 V filament bias was utilized for achieving the charge compensation. Hybrid lens mode, 40 eV pass energy and a slot aperture were applied for recording high-resolution core level spectra, Voigt profiles (singlets or doublets) being used in their deconvolution according to previously described methods 48 . The wetting properties were assessed by measuring the static contact angle (CA) with a Drop Shape Analysis System, model DSA100 from Kruss GmbH. The surface free energy (SFE) with its polar (γ p ) and dispersive (γ d ) components for the solid surface was evaluated using the apparent CA values measured between a test liquid and the sample surface. Water and diiodomethane were used as test liquids for assuring an accurate evaluation of the polar and dispersive components of the surface free energy of the investigated samples. Thus, a droplet from the test liquid was placed on the sample surface via a blunt-end, stainless steel needle (outer diameter of 0.5 mm) attached to a syringe pump controlled by the DSA3 ® software supplied with the instrument. The volume of the droplet was 1 μL for diiodmethane (DIM) while that for water (W) was increased from 1 to 5 μL (in some cases, the water droplet did not stick to the surface in order to measure the CA so the drop volume was gradually increased until this adheres to the surface). CA was measured by fitting a polynomial equation of second degree or a circle equation to the shape of the sessile drop and then calculating the slope of the tangent to the drop at the liquid-solid vapor interface line. The detailed processes of the behavior of the water droplet adhesion at the sample surface was recorded by a video camera tilted at 2-3° with respect to the plane of the sample surface supporting the droplet. Hence, the surface free energy was estimated based on the Owens-Wendt  (1 + cosθ), where θ is the CA of liquid on investigated solid surface 51,52 . The measurements were performed in duplicate at room temperature, the CA mean value being used for evaluating the surface free energy and the adhesion work. Roll-off angles were measured with a goniometer in order to control the tilt angle, the orthoscopic images being taken with a commercial photocamera.
The antibacterial activity was evaluated against non-pathogenic Escherichia coli (E. coli ATCC 25922) strain (Gram-negative bacteria) and non-pathogenic Staphylococcus aureus (S. aureus ATCC 25923) strain (Grampositive bacteria), both in planktonic state. Thus, 2 ml tryptic soy broth (TSB) and 20 μl microbial suspension with 0.5 McFarland density (1.5 × 10 8 CFU/mL) were added on the sterilized investigated samples placed in 6-well www.nature.com/scientificreports/ plates. After the incubation (carried at 37 °C for 24 h), decimal dilutions obtained from the suspension recovered in sterile saline were seeded in triplicate (3 replicates of 10 µl each) on agar medium for quantifying the colony forming units (CFU)/ml. It has to be noted that each replicate was performed in triplicate, the experiments being repeated 2 times in triplicates. The antibacterial efficiency of the investigated samples was calculated in terms of percentage of bacterial cell reduction (R, %) determined as follows: where CFU c and CFU p represent the numbers of CFU/ml for the control and each investigated sample.

Results and discussion
The surface morphology of the metallic substrate after each functionalization step was analyzed by FESEM, the images being presented at different magnifications (Figs. S1 and 2). At lower magnification (area having width × height of ~ 250 × 160 µm), the FESEM images ( Fig. S1) reveal that after each step, the zinc foil is uniformly covered by nanowires. At higher magnification, the FESEM images ( Fig. 2) indicate that the deposition parameters employed in the RF magnetron sputtering and thermal vacuum evaporation were adequately chosen in order to preserve the typical cylindrical form and the high aspect ratio of the nanowires during each preparation step. Thus, after thermal oxidation in air, the ZnO nanowires are densely grown on the Zn foil surface having the diameters down to 30 nm and the lengths varying from several micrometers to tens of micrometers. Further, the deposition of a www.nature.com/scientificreports/ granular nanostructured uniform CuO layer on the surface of the ZnO nanowires by RF magnetron sputtering results in an increase of their diameters from ~ 30 nm to ~ 60 nm, the thickness of the CuO shell being estimated at ~ 15 nm. Then, the decoration of ZnO-CuO core-shell nanowires with Ag nanoparticles by thermal vacuum evaporation leads in the deposition of metallic aggregates with irregular shape and sizes of ~ 40 nm, the thickness of the nanowire segments containing ZnO core-CuO shell-Ag nanoparticles increasing up to ~ 70 nm. The cross-sectional FESEM images (Fig. 3) emphasize the formation mechanism of the ZnO nanowires by thermal oxidation in air of zinc foil at 500 °C (temperature between the melting point of Zn (420 °C) and boiling points of Zn (907 °C)) based on a liquid-solid process in which the liquid Zn reacts with the oxygen molecules from air to form solid ZnO nanoclusters that grow via surface diffusion of Zn 53 . As can be seen in Fig. 3, the ZnO nanoclusters are assembled in a nanostructured ZnO film featured by a thickness of ~ 1 µm at the end of the thermal oxidation process. Hence, the formation mechanism of ZnO nanowires involves: (i) the formation of a thin layer of liquid Zn on the surface of the Zn substrate; (ii) the adsorption of the oxygen molecules on the surface of the liquid Zn in order to form solid ZnO nanoclusters, which further act as seeds in the growth of the ZnO nanowires; (iii) the surface diffusion of Zn ions along the side wall of the nanowire, their reaction with oxygen resulting to a simultaneous growth in both the axial and radial directions of the nanowires (the significant amount of Zn ions incorporated into the nanowire near the root region to the tip explaining why most of the nanowires become thinner at the tip).
The elemental distribution and the composition of the ZnO nanowire arrays and Ag nanoparticles decorated ZnO-CuO core-shell nanowire arrays samples were assessed by EDX spectroscopy, the EDX mapping images and the corresponding spectra being given in Fig. 4. www.nature.com/scientificreports/ The EDX mapping images illustrate an uniform distribution of the chemical elements on the investigated surfaces. The EDX spectrum of ZnO nanowires reveals the presence of Zn and O elements while the EDX spectrum of Ag nanoparticles decorated ZnO-CuO core-shell nanowire arrays discloses the simultaneously presence of the signals associated to Zn, Cu, Ag and O elements. Based on the EDX analysis, the atomic percentage of the elements contained in the analyzed samples was estimated at ~ 44-46% for Zn, ~ 2.5% for Cu and ~ 0.4% for Ag.
The structural and optical properties of the investigated samples were evaluated from data shown in Fig. 5. The XRD patterns of all samples are similar exhibiting the diffraction peaks indexed to the hexagonal wurtzite ZnO structure (ICDD 00-035-1451) and some diffraction peaks associated to the underlying Zn substrate. In the XRD patterns of the ZnO-CuO core-shell nanowires with or without Ag nanoparticles, the diffraction signature of the CuO and Ag had not been detected. In the reflectance spectra, two strong decreases can be observed one below ~ 400 nm for all samples containing nanowires and another below ~ 850 nm for ZnO-CuO core-shell nanowires with or without Ag nanoparticles due to the band-to-band transition in ZnO and CuO, respectively, the results being in agreement with those previously reported 5,6 . Additionally to these decrease peaks, the reflectance spectrum of Ag nanoparticles decorated ZnO-CuO core-shell nanowire sample discloses a shoulder of ~ 550 nm, which can be associated to the Ag plasmon band 54,55 . The presence, position and intensity of this band is strongly influenced by parameters of the Ag nanoparticles (size, shape, inter-particle distance, surface chemistry, surrounding environment, etc.) 56 . The photoluminescence spectrum of ZnO nanowires exhibits two emission bands: one intense and sharp in the UV domain centered at ~ 380 nm of excitonic origin 57 and another one weak, broad covering almost all the visible range centered at ~ 530 nm originating from defect emission 57,58 . Although, the origin of this visible emission is still under debate, the generally accepted hypothesis considers the oxygen vacancies responsible for the appearance of this "green emission" 59,60 . It has to be noted that the presence of the oxygen vacancies can modify the wetting properties of the ZnO nanowires 21 . Moreover, a study focused on the origin of the "green emission" in ZnO nanostructures (including nanowires) prepared by a vapor phase transport method (also a dry preparation technique) emphasized that the oxygen gas concentration used during the ZnO growth influences the depth of the oxygen vacancies related defects 60 .
The photoluminescence spectra of ZnO-CuO core-shell nanowires with or without Ag nanoparticles show a decrease in the intensities of these two emission bands. The result can be explained considering that the CuO layer deposited on the surface of ZnO nanowires leads to a passivation effect reducing the ZnO surface oxygen related defects 61 and to a separation effect of the charge carriers due to the formation of type-II band alignment between the two metal oxides 62,63 . (v) at ~ 532 eV to C=O due to a contamination of the surfaces with carbonate from the environment. Further, the deconvolution obtained by Voigt profiles of the core level spectra of Zn 2p, Cu 2p, Ag 3d and O 1S (Fig. S2) were analyzed for evidencing the presence of other species of these elements, the following information being (iii) Ag 3d 5/2 core level-a broad peak with two components, one centered at 368.1 eV associated to the free metallic silver atoms (Ag (0)) 66,67 and other centered at 368.7 eV related most probably to the positively charged silver atoms (Ag (0) + δ) resulted from the slight oxidation in air of the metallic atoms in the ambient conditions 68,69 or from the silver interaction with a relatively high electronegative atom like oxygen) [70][71][72] . Consequently, the XPS results prove the presence of ZnO, CuO and Ag in the Ag nanoparticles decorated ZnO-CuO core-shell nanowires samples. Both ZnO nanowires and Ag nanoparticles decorated ZnO-CuO core-shell nanowires samples were investigated by TEM measurements in order to emphasize the core-shell morphology of the ZnO-CuO nanowires and their decoration with Ag nanoparticles, the data being shown in Fig. 7.
Hence, the TEM images of a single ZnO nanowire and a single Ag nanoparticles decorated ZnO-CuO core-shell nanowire evidence a cylindrical shape for both types of nanowires with diameter of ~ 30 nm and ~ 70 nm, respectively, in agreement with the FESEM images (Fig. 2). The SAED patterns certify a crystalline wurtzite structure for ZnO nanowire (in accordance with the XRD data from Fig. 5a) and three crystalline structures for Ag nanoparticles for Ag nanoparticles decorated ZnO-CuO core-shell nanowire: hexagonal wurtzite for the ZnO core, monoclinic for the CuO shell and face-centered cubic. The EDX elemental mapping of a single Ag www.nature.com/scientificreports/ nanoparticles decorated ZnO-CuO core-shell nanowire proves the formation of the core-shell morphology by the presence of Zn K in the inner part and Cu K in the outer part. In addition, the STEM image and EDX spectra in STEM mode acquired in two areas of a single Ag nanoparticles decorated ZnO-CuO core-shell nanowire (Fig. S3) emphasize the presence of both Ag nanoparticles distributed uniformly on the surface of the nanowire and Ag aggregated nanoparticles randomly positioned along the surface of the nanowire. Usually, the wettability of a solid surface is decided by the geometrical structure of the surface as well as its chemical composition. Hence, in the present case, the micro/nano-structuration of the metallic surface and the material-dependent surface free energy have a combined and significant effect on the surface wettability, this being a key parameter related to their potential applications in surfaces with antibacterial activity. Thus, the samples were investigated by CA and SFE measurements using water and diiodomethane as test liquids in order to evaluate their wetting properties, the obtained values being given in Fig. 8 and Table S1, respectively. The FESEM images from Figs. S1 and 8 confirm that, in all cases, the metallic foil is complete and uniformly covered with nanowires on large areas. Concerning the CA values, for water, the WCA value increases from 103° (P0) to 134° (P1, P2 and P3 samples) while for diiodomethane, the DIMCA increases from 59° (P0) to 75° (P1) and then decreases to 32° (P2 and P3). Consequently, regardless their chemical composition, all samples containing nanowires on their surfaces reveal a hydrophobic behavior.
It has to be noted that in the case of ZnO nanowires, the hydrophobic property is in accordance with that reported in the literature 19,20 and even if the oxygen vacancy defects can modify their wetting properties 21 , it can be assume that the nanowire shape of the structures presented on the surface of the zinc foils plays the major role in their hydrophobicity taking into account that all samples containing nanowires have WCA of ~ 134°. Regarding the SFE components, the dispersive component is higher than the polar one for all samples. Thus, the lower polar component values (P1, P2 and P3 samples) leads to high WCA value for water droplets while the higher dispersive component (P2 and P3 samples) results in small DIMCA value for diiodomethane droplets.
Further, the adhesion of the water droplets to the surface of the investigated samples were assessed, the images being given in Fig. 9. Also, in the case of water droplets, the W ad values were estimated being presented in Table S1. It is known that when the drop slides on a sloping surface, the gravitational force can be considered a critical force to guide the movement of the drop, the energy required to roll/move the drop over a certain distance depending on the microscopic W ad . Hence, the W ad can be a measure of the contact strength between two adjacent phases, being defined as the work required to separate these two phases (ex. liquid-solid phase boundary). Thus, the micro/nanostructured effect of the metallic surfaces consisting in the presence of the nanowire structures results in a lower W ad value (~ 22 mN/m) in comparison to the smooth native metallic surface (~ 55 mN/m). It can www.nature.com/scientificreports/ be seen that the water droplet is highly adherent to the surface of the foils containing ZnO nanowires (similar to the water droplet behavior on the surface of Zn foil), resting stick, firmly pinned on the surface even when these were turned upside down. Instead, in the case of the foils having the surfaces covered with ZnO-CuO core-shell nanowires or Ag nanoparticles decorated ZnO-CuO core-shell nanowires, the water droplets rolled off very easily at slight tilt, the roll-off angle being evaluated at 55° for P2 or 25° for P3. The result can be explained taking into account the models concerning the three main states of a liquid droplet penetration in the textured (micro/ nanostructured) surface: Wenzel, Cassie-Baxter and an intermediate state between them 73 . Hence, the wetting regime for P1 sample can involve a stronger adhesion of the water droplet to the textured surface-the droplet is pinned to the surface as the air pockets are penetrated by the water while the wetting regime for P2 and P3 samples can presume a lower adhesion of the water droplet to the textured surface-the Figure 8. FESEM images (a-d) and optical photographs of the water droplets shape (a 1 -d 1 ) and diiodomethane droplets shape (a 2 -d 2 ) on the surface of Zn foil (a,a 1 ,a 2 ), ZnO nanowires (b,b 1 ,b 2 ), ZnO-CuO core-shell nanowires (c,c 1 ,c 2 ) and Ag nanoparticles-decorated ZnO-CuO core-shell nanowires (d,d 1 ,d 2 ). The contact angle is presented as the mean value. www.nature.com/scientificreports/ droplet sits on the top of the surface due to the air trapped underneath, in the gap between the nanostructures, preventing the penetration of the water. Such surfaces with hydrophobic behavior and low water droplet adhesion can be effective for antibacterial uses. In the following, the antibacterial activity of the nanostructured surfaces based on nanowire arrays was investigated against E. coli and S. aureus, the data being shown in Fig. 10a,b. The reduction in the bacterial growth is ~ 99.99% regardless both bacteria and nanowire composition types. Furthermore, no bacteria was found in the tests carried on S. aureus and Ag nanoparticles decorated ZnO-CuO core-shell nanowire arrays. Hence, it can be assume that the nanowire shape plays the significant role in achieving the excellent antibacterial response, the needle tip of the nanowire structures mechanically damaging the bacteria cells like a thorn that penetrates the membrane as can be seen in the FESEM image from Fig. 10c and in its detail. In addition, the hydrophobic and low water adhesion properties (in the case of P2 and P3 samples) can positively influence the antibacterial effect by limiting the contact area between the microbial suspension and the nanostructured surface. Still, various mechanisms can also assist the bacteria death. In our previous paper focused on the fabrication of the water stable photocatalysts based on ZnO-Cu x O core-shell nanowires 16 , we evidenced that a ZnO dissolution can occurs in the aqueous solutions and the presence of a CuO layer with an adequate thickness on the surface of the ZnO nanowires protects them against the dissolution in the aqueous media. Accordingly, the zinc ions (P1 sample) or the silver ions (P3 sample) generated in the aqueous media can adhere to the cell wall, increasing its permeability and further leading to the cell membrane rupture 74 while the formation of ZnO-CuO p-n junction 16 (P2 sample) and Ag-CuO Schottky barrier 75 (P3 sample) can favor the separation of charge carrier pair, which further can improve the generation of highly ROS involved in the mechanism of bacteria killing. Consequently, the antibacterial activity of the samples due to the mechanically damaged bacteria membrane can be assist by a chemical contact killing bacteria mechanism.
The outcome of the experiments proves that such functional nanostructured surfaces based on silver nanoparticles decorated ZnO-CuO core-shell nanowire arrays with low water droplet adhesion and excellent bactericide effect can open new insights in the field of water repellent and antibacterial coatings.

Conclusions
High-aspect-ratio silver nanoparticles decorated ZnO-CuO core-shell nanowire arrays were fabricated by combining low-cost and reproducible dry preparation techniques that can be relative easily scaled to large area. Hence, on zinc foils, ZnO nanowire arrays were grown by thermal oxidation in air, subsequently coated with a CuO layer by RF magnetron sputtering, the obtained ZnO-CuO core-shell nanowires being further decorated with Ag nanoparticles by thermal vacuum evaporation. The wettability measurements show that all investigated samples reveal hydrophobic behavior. Moreover, the native Zn foil and the ZnO nanowires arrays are featured by a high water droplet adhesion while ZnO-CuO core-shell nanowires arrays with or without Ag nanoparticles by a low water droplet adhesion. The antibacterial tests carried on E. coli and S. aureus emphasize that all nanostructured surfaces based on nanowire arrays present excellent antibacterial activity against both type of bacteria, the hydrophobic and low water adhesion behavior of the nanowire samples enhancing the antibacterial effect by limiting the contact area between the microbial suspension and the nanostructured surface. The nanowire shape plays the major role in achieving the excellent antibacterial response, the nanowire structures mechanically damaging the bacteria cells, process that can be assists by other mechanisms involving release of metal ions or formation of different type of junctions. This study emphasized that such functional surfaces can be viable candidates for water repellent surfaces with enhanced antibacterial activity.

Data availability
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